The present invention generally relates to a liquid crystal display (LCD) and its fabrication method, and more specifically to a liquid crystal display that can be fabricated on a silicon wafer and its fabrication method.
The ever-increasing demand of high quality images with SXGA or UXGA resolution in current display markets brings tough challenges to the liquid crystal display industry since it is necessary to shrink the pixel size and keep a sufficiently high aperture ratio at the same time. In order to meet both requirements, the inter-pixel gap between pixel electrodes must be reduced accordingly. However, when the inter-pixel gap is reduced to a size less than the cell gap, the fringe field between the on-pixel and the adjacent off-pixel will cause a number of undesired effects on the image quality and electro-optical properties of the liquid crystal on silicon (LCoS) devices. For example, it will induce the generation of disclination lines within the on-pixel as a result of the competition between two topologically inequivalent domains. The disclination lines will influence the electro-optical properties of a liquid crystal cell in both normally black (NB) and normally white (NW) cases. In the normally black case, they appear dark within the bright on-pixel and reduce the effective aperture ratio and the reflectance. In the normally white case, however, they degrade the dark state and reduce the contrast ratio.
FIG. 1a shows the calculated reflectance of liquid crystal cells in a conventional vertically aligned liquid crystal display when one pixel is on and the adjacent pixels are off. The horizontal axis represents the pixel position while the vertical axis represents the percentage of the reflectance. The wavelengths of three incident lights are respectively 500 nm, 550 nm and 600 nm. The pixel size is 15 xcexcm. The total width of the image displayed in FIG. 1a is 24 xcexcm which covers three pixels. The width of the center bright on-pixel shown is 15 xcexcm and the width of each adjacent off-pixel plus the inter-pixel gap is 4.5 xcexcm. The electrical voltage applied to both adjacent off-pixels is different from that applied to the center on-pixel. As shown in FIG. 1a, the disclination line appears dark in the bright on-pixel and part of the adjacent off-pixels is turned on.
Similarly, FIG. 1b shows the calculated reflectance of liquid crystal cells in a conventional twisted nematic (TN) liquid crystal display when one pixel is on and the adjacent pixels are off. The pixel size is 15 xcexcm. The total width of the image displayed in FIG. 1b is 30 xcexcm. As shown in FIG. 1b, the disclination line also appears dark in the bright on-pixel and part of the off-pixels is turned on.
Nowadays, the range for the size of the pixel fabricated on the LCoS wafer is as small as 10 to 15 xcexcm and the gap between pixels is about 1 xcexcm. Therefore, the fringe field among pixels is very large. Frame inversion is generally used as a driving method to increase the reflectance. Although this driving method can get higher reflectance, there are disclination lines generated when the voltage difference between two pixels is high. This reduces the reflectance, slows down the response speed and provides a poor image quality.
In a conventional LCoS light valve, a transparent indium-tin-oxide (ITO) and a metal material with high reflectivity, such as aluminum, are used as pixel and common electrodes. Because the work functions of two electrodes are different, it is a common practice to apply a constant dc voltage to the ITO common electrode to compensate for the difference in the work function. However, experimental observations indicate that no single constant dc voltage can eliminate flickers and image sticking across the light valve.
This invention has been made to overcome the above-mentioned annoying issue of disclination lines caused by the fringe field between the on-pixel and the adjacent off-pixel of a conventional liquid crystal display. The primary object is to provide a liquid crystal display fabricated on a silicon wafer, called LCoS device. Another object of the present invention is to provide a method for manufacturing the LCoS device.
The LCoS device comprises an upper glass substrate, and a lower silicon wafer substrate with driving circuits and active switching elements formed thereon. A layer of pixel electrodes having multiple pixel electrodes is formed on the silicon wafer substrate. The active switching element on the silicon wafer substrate is used to control the voltage applied to the pixel electrodes. An insulation layer is deposited on the layer of pixel electrodes and a common electrode layer is then formed above the insulation layer. The common electrode layer has a plurality of major common electrodes each being formed above the gap between two adjacent pixel electrodes. Two alignment films are printed respectively on the inner side of the upper glass substrate and on the common electrode layer. A liquid crystal layer is filled with negative type liquid crystals between the alignment films.
According to the invention, the common electrode layer may further comprise minor common electrodes equally spaced and interposed between every two adjacent major common electrodes. The particular geometry of common electrodes generates optimized fringe field that is capable of eliminating the annoying issue of disclination lines and keeping high reflectance in the modulated area. In practice, the top of the common electrode layer may or may not be covered with a protecting layer which is made of a high optical transmittance material.
In a preferred embodiment of an LCoS device according to the present invention, both pixel and common electrodes can use the same metal material, such as aluminum. Therefore, both electrodes have the same property and the image quality is improved. Negative type liquid crystal material is used in the invention. Simulation has been performed with varying cell gap, pre-twist angle, and applied voltage by using four driving methodsxe2x80x94frame, column, line, and dot inversion. The simulation results on the LCoS device of the invention show that the worst-case performance of reflectance of the on-pixel is quite high with average value over 75% while keeping the off-pixel in a perfect dark state.